In this laboratory we have developed an embedding medium for ultrathin sectioning which depends upon the polymerization of glutaraldehyde with urea, in the presence of substantial quantities of water. The highly polar character of these molecules permits the retention of lipids in ultrathin sections suitable for electron microscopy. These embedments also maintain normal spatial relations dependent upon glycocalyx gels. Various staining procedures then can be used. This is the basic methodology we are employing to study the surfactant system of the lung. Presently the principal focus of attention is on the manner of accumulation and storage of phospholipids in the secretory bodies of the Type II pneumocytes, and the mechanism for the dispersion of the secretory product in the alveolar spaces of the lung. A second investigation which is dependent upon essentially the same techniques concerns specific details associated with glomerular capillaries. Of particular interest is the sieve-like, ordered structure previously described as existing in the slit pores between podocyte processes by Rodewald and Karnovsky (1974). Insofar as this is a specialization of the glycocalyx, we hope that the new techniques will permit a substantial extension of information concerning this. An aspect of preserving its highly ordered pattern relates to the manner of the initial fixation, and following Rodewald and Karnovsky's lead, we are exploring fixatives that include tannic and gallic acids and other related compounds. BIBLIOGRAPHIC REFERENCES: Nir, I. and D.C. Pease. Substructure in rod photoreceptor membranes. Anat. Rec. 182: 15-28, 1975. Pease, D.C. The identification of phospholipid micelles in glutaraldehyde-urea embedded tissue. 33rd Ann. Proc. Electron Microscopy Soc. Amer., Las Vegas, Nevada, 1975, pp. 494-495.